Photosensitive means for detecting the position of radiating or reflecting bodies



March 8, 1966 H. GABLOFFSKY 3,239,672

PHOTOSBNSITIVE MEANS FOR DETECTING THE POSITION OF RADIATING ORREFLECTING BODIES Filed Feb. 12, 1959 6 Sheets-Sheet 1 1 2 P 12 35 g asm n n n n n n m at; 40--- SPACE FREQUENCY F. 4 29" 3 J l' 5l 6 SD": 5552 P4 (4 DRn/E I RECORDER I 54 55 7 i 59 I OPTIC5 DETECTOR FHJTER T CELLa G} I 0 4-9 I L 5 u f SEARCH MECHANKAL (PRIOR ART) PROGRAM Dmva FF. 5

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7,4 M 23 5 57 (man ART) BY 5 (Mu-3., z. J Man. a" A March 8, 1966 H.GABLOFFSKY 3,239,672

PHOTOSENSITIVE' MEANS FOR DETECTING THE POSITION Filed Feb. 12, 1959 OFRADIATING 0R REFLECTING BODIES 6 Sheets-Sheet 2 SPmDRuva 69 POWER iSUPpLy j 05 7 S'Pm 67 75 78 DR\VE 2 r MOTOR l l 79 I I ODTiCAL c CELL 5'COLLECHON k I APPARATUS l I 92 95 -52; 96 I PLANETAR DRWE. 97 O5 MOTOR IOz REFERENCE.

SKGNAL \O4 R REFERENCE DRIVL GENERATO SKBNAL DOWER SUPPLY Him L GABLapps y INVENTOR. ELM 227M A7TOIENE y March 8, 1966 H. GABLOFFSKYPHOTOSENSITIVE' MEANS FOR DETECTING THE POSITION OF RADIATING ORREFLECTING BODIES 6 Sheets-Sheet 3 Filed Feb. 12, 1959 5 m Z W l1 rC TI'OMDUTER wm c l I W. w 5 r K R R m H m w m 08% T T OI. u U5 1 v N S R E NMm WW M E MA mm Mm w SK 2W W 2 m J ma ..l a Z i R QR rm (a m n FELEVATION DRWE (PRlOR ART) 2'? W W J A77'ORNEY March 8, 1966 H.GABLOFFSKY PHOTOSENSITIVE MEANS FOR DETECTING THE POSITION 0F RADIATINGOR REFLECTING BODIES Filed Feb. 12 1959 2 11 (PRIOR ART 6 Sheets-Sheet4.

HEM/z GABLOFASK) INVENTOR.

March 8, 1966 H. GABLOFFSKY 3,239,672

PHOTOSENSITIVE MEANS FOR DETECTING THE POSITION OF RADIATING ORREFLECTING BODIES Filed Feb. 12, 1959 6 Sheets-Sheet 5 ///Nz GA BLOFFS yINVENTOR.

TTORNEY March 8, 1966 GABLOFFSKY 3,239,672 PHOTOSENSITIVE MEANS FORDETECTING THE POSITION OF RADIATING OR REFLECTING BODIES 6 Sheets-Sheet6 Filed Feb. 12, 1959 SUE 3 4 20.55 60 ONEZM WON MN WW 3,239,672PHOTOSENSITIVE MEANS FOR DETECTING THE POSITION OF RADIATING ORREFLECTING BODIES Heinz Gabloffsky, Torrance, Calif., assignor, by mesneassignments, to The Bunker-Ramo Corporation, Canoga Park, Calif., acorporation of Maryland Filed Feb. 12, 1959, Ser. No. 792,920 1 Claim.(Cl. 250-203) This invention relates to apparatus for use in detectingthe existence of radiating or reflecting bodies and/ or their angularpositions, and, more particularly, to means for developing an electricalsignal representing the position of a body radiating energy in thevisible and/or invisible light spectrums.

In the prior art, numerous systems have been disclosed for detecting anddetermining the position of bodies from which is emanated some form ofdetectable energy such as light heat, or radio frequency waves. A numberof these prior art systems have provided considerable sensi-' tivity andaccuracy in their operation. However, especially in the field of visibleor infrared target detection, there exists considerable need forimproving the sensitivity and response speed of such systems so as toafford more suitable apparatus, by way of example, for detecting ortracking moving bodies or targets such as modern supersonic aircraft andmissiles.

As will appear hereinafter, although the present invention findsparticularly useful application to detection systems responsive toinfrared radiation, the novel features thereof are also of advantage inradiant energy detection systems based upon the detection of radio wavesand visible and invisible light rays. To this end, the term optical,often employed as descriptive of visible light processing systems, will,as used in the specification, be construed as'being also descriptive ofsystems for collecting, directing, refracting, transducing and detectingradiant energy other than that constituting visible light.

In most prior art optical systems employed for detecting and determiningthe position of a target, the space in which it is suspected that anenergy emanating target may be present is systematically examined by anoptical type energy collection apparatus. The energy collectionapparatus, generally employing combinations of mirrors and lenses, isdesigned to be responsive on a selective basis to only that energy whichis collected within a given angular field of view so that the collectionapparatus may be regarded as having a response pattern generallyrepresentable as a solid cone extending into space with the apex of thecone positioned at a given point of observation. This angular field ofview, or response pattern, is generally referred to as the instantaneousfield of view or sometimes field of view of the collection apparatus andis defined by the size of the field stop characterizing the collectionapparatus itself. The field stop size of such collection apparatus isgenerally determined either by a diaphragm restriction in the opticalpath within the apparatus or by inherent characteristics of the lensesor mirrors used. The optical axis of the collection apparatus, asprojected into space, is, in most cases, centrally disposed with-in thisinstantaneous field of view so that the optical axis of the energycollection system is in geometric coincidence with the axis of theconical response pattern of the apparatus. The energy collected withinthe instantaneous field of view is directed to an energy sensitive cellwhich develops an electrical potential or signal, the magnitude of whichrepresents the intensity of the total radiant energy collected withinthe field of view which includes energy emanating from the target per seas well as background radiation A United States Patent such as sky,clouds, water, etc., against which the target may appear.

However, in accordance with the prior art technique, it is common tofind that a circular disc-like chopping reticle is positioned within theenergy collection apparatus at an image or focal plane thereof. Such areticle is rotated about its axis in interrupting relation to radiationcollected by the apparatus to chop the radiation as it is directed tothe sensitive cell. This type of reticle is generally called a choppingreticle because it comprises a pattern of carefully dimensionedalternate areas of relative opacity and tran'smissivity to energycollected by the apparatus to intercept radiation. These areas oftenhave the shape of sectors of a circle. The areas of transmissivity,defined by alternate areas of relative opacity on the reticle, aresometimes called reticle apertures. It has been the practice to alignthe rotational axis of the reticle with the optical axis of thecollection apparatus as at an image plane thereof so as to focus orimage the field of view on the reticle. The field of view, as imaged onthe reticle, is generally circular in shape and is defined in size bythe aforementioned field stop of the apparatus. The diameter of thereticle has in the past been made just large enough to embrace theentire imaged field of view to thus interfere with all energy reachingthe cell.

In prior art systems incorporating such chopping reticles, the reticleis rotated about its axis at a selected angular velocity. As it rotates,the reticle apertures move across the imaged field of view and modulatethe total energy passing therethrough to the energy sensitive cell. Thecell then produces an output signal having a directcurrent componentproportional to the average illumination thereof and generally aplurality of alternating-current harmonically related modulationcomponents, the largest and fundamental alternating-current componenthaving a frequency equal to the chopping frequency of the reticle. Themagnitude of the modulation of the energy radiated from targets orimages in the imaged field of view by a chopping reticle and hence themagnitude of the corresponding fundamental alternating-current signalproduced by the cell will be a maximum only for targets having the sameorder of dimensions as the reticle apertures themselves in the directionof movement thereof. This is true because only a relatively smallportion of the energy radiating from larger or smaller targets will bemodulated continuously by the reticle chopping action, Whereas the totalenergy received from a target having a linear dimension equal to that ofa reticle aperture will be modulated. Due to the fact that energyradiating from targets of a predetermined size are modulated to agreater extent than larger or smaller ones, a chopping reticle thuseffectively discriminates against targets of the larger and smallersizes in favor of those of the predetermined size. In other words, itexhibits a certain size selectivity as an electrical filter exhibits acertain time-frequency selectivity. Analogously then, the target sizediscriminating effect of a chopping reticle is called space filteringbecause the maximum contribution of targets to the fundamentalalternating-current component of the cell output signal is limited by achopping reticle to targets of a certain size. In practice, it isdesirable to make the width of the reticle aperture substantially equalto the blur circle of the optical system. The blur circle is the minimumsize to which any size target can be focused on the reticle due toinherent aberrations in the mirror and lens elements of the opticalsystem.

In practice, the detection of and determination of the position of agiven target with apparatus including a chopping reticle is accomplishedin two steps, usually termed search and track. First, in search, thecol- "ice lection apparatus is mechanically driven to execute asystematic scanning action which results in the exploratory examinationof a volume of space which is many times greater than the instantaneousfield of view subtended by the collection apparatus, and in which it isexpected that an energy radiating target may be present. The output ofthe energy sensitive cell is oftentimes recorded or stored, on a memorybasis, as the search action proceeds, so that after completion of thesearch cycle the apparatus may be automatically returned to one or moreselected positions corresponding to the orientations of the apparatus atthose specific instances within the period of the searching cycle atwhich target energy has been detected. After redirection of theapparatus so that its field of view embraces that general volume inspace in which a specific target has been detected, the second or trackstep of the position determining process is initiated, namely, that ofdetermining the angular position of the target with respect to theoptical axis of the energy collection apparatus during which time theoptical axis is often moved or nutated around a circular path in spacewhich embraces the target.

During nutation, the optical axis of the energy collection system, asprojected into space, is moved around a closed loop or path defined on aspherical surface in space. This path is so positioned and restricted insize as to afford pick-up of energy from the target during the movementof the instantaneous field of view. When such is the case, a frequencymodulation will be imposed on the output signal of the radiant energysensitive cell. By comparing the phase of the frequency modulatingsignal of the cell output signal with a signal having a phaserepresenting the position of the optical axis as it is nutated, thepolar angle coordinate of a target in the imaged field of view may beascertained. Similarly, the magnitude of the frequency modulating signalwill be proportional to the polar radius coordinate of the target in theimaged field of view. From this information, a servo control system maybe brought into action to track or follow any target motion.

Although both searching and tracking may be performed with the reticleaxis coinciding with the optical axis of the collection apparatus, thereis a distinct disadvantage in such systems due to the inherent poorspace filtering characteristics of spoked type reticles. Specifically, aconventional spoked reticle is far from an ideal space filter becausethe distance between relatively opaque areas defining the reticleapertures varies with the radial distance from the reticle center.Although some space filtering is performed by centered spoked reticles,undesired signals, generally called noise, are produced by the sensitivecell at the fundamental chopping frequency of such a reticle. Maximumcontribution to this noise is made by noise sources presenting images inthe imaged field of view equal in size to reticle aperture spacing atthe particular radial distance from the reticle center that they exist.The effect of plural noise sources is then cumulative and all radiantenergy sources such as clouds, sun, terrain, etc., which comprisebackground radiation against which a target may appear, are potentialnoise sources. The problem of distinguishing a target signal from noiseproduced by the above-desired sources is an unusually difficult one. Dueto the fact that such noise appears as signal energy at the fundamentalchopping frequency of the reticle, an electrical filter simply cannot beused to discriminate against this type of noise.

A distinct disadvantage is encountered by employing conventionalcentered chopping spoke reticles in search. As the axis of the opticalsystem is translated in any rectilinear pattern, a Doppler frequencysignal of a lower frequency is generated by passage of a point target onone side of the reticle center and one of a higher frequency by passageof a point target on the opposite side of the reticle center. In thefirst case, scanning is performed in the same direction as reticlerotation and in the second case in the opposite direction. This meansthat the effective chopping frequency of the system may be shifted overa relatively large frequency band depending upon the position of thetarget with respect to the axis of the reticle. This means that thebandwidth of the signal transducing system following the energysensitive cell must also have a corresponding relatively largebandwidth. This decreases the signal to noise factor of the system andreduces its effective selectivity.

Tracking performed by nutation of prior art optical systems also hasseveral large disadvantages. In the first place, during nutation theentire optical collection apparatus is moved in space around the target.Since the aberration or distortion imposed on the target changesthroughout the field of view, the effective size of target image willchange during nutation because the effective blur circle of the systemis different at difierent points in the field of view. Thus, with agiven sized reticle aperture, the amplitude of the signal developed bythe cell will change during nutation. Thus, again an undesirable resultis produced by this variation which is similar to the rather poor spacefiltering of spoked reticles.

Prior art tracking systems also continually suffer from high levelscanning noise. This is because the entire background over which thefield of view of the collection apparatus is scanned in nutation mayencounter widely varying radiation intensities. Thus, the intensity ofradiation intercepted by the energy sensitive cell will be modulatedsimply by virtue of the scanning motion of the energy collectionapparatus and undesirable signals will be generated other than thoseproduced by point targets. Since, by general definition, any signalsexcept point targets are considered noise and are considered undesirablebecause special means must be provided to discriminate against them,scanning noise is, therefore, a definite disadvantage of prior arttracking systems.

Still further, in prior art centered reticle tracking systems anon-linear representation of target position is also encountered duringnutation when a target falls outside of the instantaneous field of viewof the collection apparatus or when the image of the target happens tofall on the rotational axis of the reticle. This is true because of thecomplete loss of any target representing modulation during this periodof time.

Still another serious disadvantage of prior art nutating trackingsystems is that the whole optical apparatus of the system must be moved.By nature, this optical apparatus must be both large in size andrelatively heavy. The fact that it must be nutated means that itsperformance is necessarily unreliable, due to the mechanical nature ofthe movement which must be produced. Still further, the opticalapparatus must be moved at a relatively slow rate and only by the use ofconsiderable amount of electrical power. Due to the fact that theoptical apparatus is of a substantial size, it must be made of ratherheavy materials to withstand the stresses imposed upon it by theacceleration forces causing it to nutate.

The present invention overcomes these and other disadvantages of theprior art by providing the combination of -a movable reticle sopositioned with respect to an image representing the intensityvariations within a restricted field of view sustained by an energycollection system that the apertures of the reticle scan the image withthe relative motion of all reticle apertures embraced by the field ofview, at any instant, being in substantially the same direction withrespect to a given reference line within the field of view. In trackingan object conditionally falling within the field of view, additionalmeans are provided, in accordance with the present invention, forchanging the direction with which the reticle apertures effectively scanthe imaged field of view.

The reticle employed with the invention may be of several designs, but,as stated previously, a conventional disc shaped spoked reticle may beemployed. In search,

the reticle may just be rotated about its axis and the collectionapparatus moved. In tracking, the reticle is again rotated about itsaxis but its axis, in turn, is translated around the imaged field ofview. Translation of the reticle axis itself causes a relative motionbetween the image of a target and the reticle apertures themselvesalternately in the same and in an opposite direction within the imagedfield of view. This produces a frequency modulation of the signaldeveloped by the cell from which the position of a point target in theimaged field of view may be determined. This determination may be madein exactly the same manner as is made by prior art tracking systemsutilizing nutation. As before, the magnitude of frequency modulationthus produced is representative of the polar radius coordinate of apoint target in the imaged field of view and the phase of the frequencymodulation signal with respect to a predetermined phase of the reticleaxis translation is representative of the polar angular coordinate ofthe point target in the imaged field of view. Thus, the same effect isproduced by the present invention as that produced by prior art systems,but nutation of the energy collection apparatus during tracking isobviated. For this reason, the invention has several large advantagesover systems of the prior art.

For example, the invention provides improved space filtering over thenutating prior art optical systems both in searching and trackingbecause the instantaneous field of view is so projected on the reticlethat the reticle apertures active within the field of view at all timesare of substantially the same configuration.

Due to the fact that in search, depending upon its position within theimaged field of view, a target in the prior art might pass across thereticle in the same or opposite direction to that of reticle spokechopping motion at successive times during search, a frequency shiftcould be encountered which would make it necessary to reduce theselectivity of the associated signal transducing system connected to theoutput of the energy sensitive cell. However, in accordance with theinvention, regardless of the position of the target within the imagedfield of view, the direction of chopping will always be in the samerelation to the motion of the search during any straight line portion ofthe search cycle. That is, during any given straight line pathcomprising the search sequence, a target will not be scanned in a mannersuch that the energy radiated by it will be chopped in either one or twodirections but will be chopped only in one direction. This means thatthe present invention is much more selective in searching operationsthan the conventional centered chopping spoke reticles of the prior artwhere a larger band of frequencies are generated by virtue of the factthat targets are scanned over the reticle in varying directions relativeto reticle rotation on opposite sides of the reticle center.

The well known variations in the effective size of the blur circle,"encountered by prior art centered reticle type nutating tracking systemsis avoided by the use of the invention and the resolution of pointtargets is also thus much improved over that produced by prior artdevices. This is true again because nutation of the field of view inspace is eliminated.

The scanning noise produced by prior art tracking systems using nutationof the field of view in space is also substantially reduced since thebackground of the system of the present invention is alwayssubstantially stationary. It is to be noted especially that duringtracking the instantaneous field of view in the instant case remains ina substantially stationary position, or at least it is moved relativelyslowly during tracking to keep a point target exactly at its center andon the optical axis of the collection apparatus.

Also, since during the operation of the invention, in tracking, theinstantaneous field of view does not move appreciably in space, a targetcannot be lost momentarily as it often is in nutating systems of theprior art where it may either fall outside of the instantaneous field ofview or on the reticle axis during a portion of the nutating cycle ofthe energy collection apparatus. Thus, a more linear indication oftarget position may be produced in the instant case over the entireinstantaneous field of view where this was not possible with prior artnutating optical systems.

In accordance with the invention, a nutation of the entire radiantenergy collection apparatus is completely obviated and may be maintainedsubstantially stationary except for minor movements required to actuallyfollow a target. Hence, the prior art disadvantage of nutation requiringmovement of the relatively large size and, therefore, necessarily arelatively heavy radiant energy collection apparatus is eliminated. Thereliability of the present invention is, therefore, substantiallyimproved over that of the prior art due to the substantial reduction inthe size and weight of the equipment which must be mechanically movedduring the tracking operation. Still further, the large electrical powerrequired to nutate the large and heavy collection apparatus in the priorart is obviated.

The above-described and other advantages of the invention may be betterunderstood when considered in connection with the following description.In the drawings which are to be regarded as merely illustrative:

FIG. 1(a) is a diagrammatic view of a reticle having one transparentarea therethrough adapted to pass over a circular imaged field of view;

FIG. 1(1)) is a diagrammatic view of an imaged field of view having thetransparent area of the reticle shown in FIG. 1(a) illustrated in dottedlines;

FIG. 2 is a graph of the radiant energy power which may pass through thetransparent area during movement of the reticle shown in FIG. 1(a) overthe imaged field of view shown in FIG. 1(1));

FIG. 3 is a graph of two Fourier transforms of portions of the curveshown in FIG. 2;

FIG. 4 is a diagrammatic view of one type of reticle which maybeemployed with the invention;

FIG. 5 is a diagrammatic view of a conventional radiant energy detectionsystem employing a conventional spoketype reticle on which an imagedfield of view is centrally located, with the optical axis of the energycollection systelm being coincident with the rotational axis of the etice;

FIG. 6 is a front elevational view of a spoke-type reticle employed inthe prior art;

FIG. 7 is a schematic diagram illustrating movement of an instantaneousfield of view in a search operation which may he performed by theapparatus shown in FIG. 5;

FIGS. 8a and 812 show graphs of a group of waveforms characteristic ofthe operation of the apparatus shown in FIG. 5;

FIG. 9 is a schematic diagram of a prior art radiant energy detectionsystem employing nutation during a tracking operation;

FIG. 10 is a diagrammatic view of one embodiment of the invention;

FIG. 11 is a sectional view of a conventional optical collectionapparatus diagrammatically illustrated in FIG. 10 which may be employedwith the invention;

FIG. 12 is a sectional view of an alternative embodiment of theinvention; and

FIG. 13 is a block diagram of a radiant energy detection systemincorporating the invention.

To better understand the present invention and typical operatingenvironments therefor, some consideration will first be given to severalfundamental aspects of opticaltype target detection systems embodyingchopping reticles.

For example, in the drawing of FIG. 1(a), a reticle 20 having only asingle rectangular slot 21 is shown having a symmetrical center 22. Slot21 is radially disposed from center 22 to the periphery of reticle 20.In FIG.

1(b), the slot 21 is indicated in dotted lines at 21' passing around acircular imaged field of view 23 having a symmetrical center 22 whichmay be in a position corresponding to the center 22 of reticle 20.imaged field of view 23 contains clouds 24, ground 25 having a roadway26 thereon, buildings 27 on a horizon 28 with trees 29. Also shown byway of example in FIG. 1(1)), within imaged field of view 23, is anairplane 30. In the instant case, it will be assumed that it is theairplane 30 which is to be detected, its position determined, and itsmovement tracked. For purposes of illustration, the airplane 30 is shownsubstantially larger than would be the case in typical detectionenvironments where in practice the airplane would be resolved only as acircle corresponding in size to the blur circle of the opticalcollection apparatus defining the imaged field of view 23.

Reticle 20, shown in FIG. 1(a), is constructed of a sheet materialrelatively opaque to radiant energy. However, in the sheet, therectangular aperture 21 of a width w" is provided which is relativelytransparent to radiant energy. The total illumination power P in watts,passing through aperture 21 as reticle 20 is rotated about its center 22and about the center 22' of imaged field of view 23 may be plotted as afunction of the angular displacement '7, that aperture 21 has moved froman initial vertical position, which has been arbitrarily selected. Sucha graph is indicated by a line 31 in FIG. 2. In FIG. 2, the portion 32of solid line curve 31 is shown to indicate that that portion of thefunction which would be produced in response to the airplane 30appearing in imaged field of view 23 shown in FIG. 1(b).

The relatively fiat dotted line portion 32 of the function depicted bycurve 31 is illustrated for purposes of comparison to indicate theappearance of the function when airplane 30 does not exist in imagedfield of view 23. A comparative analysis of curve 31 inclusive of eitherportion 32 or 32' may be made by what is known as the Fourier transform.As before, the symbol '7 represents the angular position of aperture 21relative to a vertical line extending from center 22 upwardly as shownin FIG. 1(a). A dotted line 33 indicating a particular angular positionof 'y is located at a point :36O.

The Fourier transform of the power versus angle functions shown in FIG.2 appears substantially as shown in FIG. 3. In considering thetransforms of FIG. 3, it is helpful to note that it is common practice,in electrical signal analysis, to express an amplitude or power versustime function in terms of a power distribution of electrical signalfrequencies or frequency distribution. For example, if the time varyingdemands of some electrical load circuit would be represented by anelectrical signal Waveform, this signal waveform, by Fourier analysis,can be expressed or transformed into an expression depicting the poweramplitude relationships between a plurality of electrical signalfrequencies. That is, a power versus time function is transformed intoan equivalent expression of power versus time-rate-of-power change. Bystudy of such a Fourier analysis or transform, it can be determined atwhat signal frequency or frequencies the largest amount of electricalpower is represented. Likewise, in connection with the power versusangle functions of FIG. 2, a Fourier transformation of these functionswill result in an expression of power versus angle rate-ofpower-change.Just as the time-rate-of-power change employed in electrical signalanalysis is expressed in cycles of power change per unit time (timefrequency or cycles per second), so angle-rate-of-power change in imagebrightness analysis may be expressed in cycles of brightness per unitangle (angle frequency). The concept of the frequency with which thepower passing through a reticle aperture changes per unit angle ofaperture displacement, gives rise to the phrase space frequency. Thus,any image of an object may be described in terms of the amplituderelation between a plurality of space frequencies. It follows then thatwhen an imaged field of view such as indicated in FIG. 1(b) containsrelatively small objects, such as the aircraft 30, the power versusspace frequency description of this field of view will indicatesubstantial power at higher space frequencies. On the other hand, thespace frequency description of larger objects in the field of view shownin FIG. 1(b) would indicate relatively less power at these higher valuesof space frequency.

With the above in mind, the transform 34 of FIG. 3 describing the spacefrequency content of the field of view in the presence of a target,shows that at higher values of space frequencies a considerable amountof power is represented. Contrariwise, the transform 35, describing allthe other larger objects in the imaged field of view in the absence ofthe target 30, represents considerably less power at these higherfrequencies. The difference between the transforms 34 and 35, of course,represents the power versus space frequency description of the target30. This description is indicated by line 36.

In accordance with the present invention, ideally, a chopping reticlearrangement equivalent to that shown in FIG. 4 is employed whichcomprises a reticle 37 having an infinite number of reticle apertures 38which movably intercept a field of view 39. In the arrangement of FIG.4, it is seen to be typical of the present invention that as the reticle37 is moved across the field of view 39 all of the reticle aperturesembraced by the field of view are moving in substantially the samedirection. Similar edges or sides of apertures 38 are spaced from oneanother by a given distance A which is appreciably less than the imagefield of view 39. If the variations in the total power passing throughthe reticle 37 over the entire imaged field of view indicated at 39thereon is examined while the reticle is moved across the field of view,it will be found that the peak to peak amplitude of such variations willbe a maximum in response to substantially only those image intensitygradients or objects whose effective dimensions, in the direction inwhich the reticle is moved, is substantially A/2. This intensity changerepresents a periodicity of intensity change, per unit distance, of A ora space frequency of l/A in cycles per unit distance, rather than cyclesper unit angular measure, with the reticle 20 shown in FIG. 1(a). Thus,roughly speaking, the action of such a reticle comprising an infinitenumber of apertures is to reinforce a particular value of spacefrequency. The action of reticle 37 of FIG. 4 is, viewed from adifferent standpoint, discriminatory in nature. That is, the reticletends to discriminate against all space frequencies other than l/A andits harmonics. If the reticle 37 were provided with only one aperture 38corre sponding to the aperture 21 of reticle 20, shown in FIG. 1(a), thecurve 31, shown in FIG. 2, would have to be changed corresponding tomovement of the reticle in the direction of arrow 40. This curve wouldhave to be modified to go to zero at a certain distance of the movementof the aperture 38, say at zero from the left-hand side of imaged fieldof view 39 to zero at the right-hand side. However, the same frequencydescription of objects may still be analogous to those illustrated inFIG. 3. However, the space frequency description of the field of view39, shown in FIG. 4, with, and without, all the background and target 30present therein, includes the effect of the reticle aperture in scanningthe finite imaged field of view 39 as an object itself. It will beremembered that the shape of the imaged field of view is defined by theaperture characterizing the energy collection apparatus. That is, thecircular imaged field of view as a whole has some value of averagebrightness. Thus, the reticle aperture, in passing over the imaged fieldof view, transmits power changes representative of an object having thesize of the imaged field of view itself. This is represented by the factthat the energy passed by an aperture 38 before it enters imaged fieldof view 39 will pass zero energy and after it has passed completelythrough imaged field of view 39 will pass zero energy. Because of thezero intensities of the extreme positions of aperture 38, the Fouriertransform of the power versus distance function, which may be similar tothat indicated at 32' in FIG. 2, as represented at 35 in FIG. 3 (thebackground alone), will have nulls n n 21 etc. These nulls correspond tospace frequencies at which the average intensity of the background, aslimited or shaped by the aperture defining the circular field of view,contributes no energy.

Thus, the reticle space A of FIG. 4 is such to reinforce a spacefrequency corresponding to a null or zero of the background 35. Thepresence or absence of a target such as aircraft 30 in imaged field ofview 39 may be quite effectively determined. Such nulls in thebackground transform are indicated at n n n n n n n etc., each nullcorresponding to a value of space frequency at which substantially nopower exists or is contributed by background content of the field ofview. Thus, any power that can be measured through the reticle 37, whenso dimensioned as to reinforce a null, must be attributable to an objecthaving dimensions comparable to that of the aircraft 30, if such ispresent in imaged field of view 39. The particular background null ofthe reticle 37 which it should be constructed to reinforce is notcritical. With a transform of the character shown in FIG. 3, however, itis expedient to choose a null defined by those portions of the curvewhose slope adjacent the null is of lesser value than slopes adjacentother nulls. Such nulls seem to appear at higher value of spacefrequencies. This reduces the precision with which the reticle aperturespacing must be dimensioned to realize the substantial percentage changein the power it transmits as a function of the presence or absence ofthe target. However, as the transform of FIG. 3 shows, the amount ofpower contributed at any given value of space frequency within the fieldof view tends to decrease as the value of space frequency is increased.Over-all system signal-to-noise considerations, therefore, suggest thata null be selected at some value of space frequency close to the spacefrequency at which an expected target contributes substantial energy. Asa compromise, therefore, between precision with which the reticleconstruction must be carried out and signal-to-noise considerations, anull such as n in FIG. 3, is, by way of example, selected to define thatspace frequency which the reticle should be designed to reinforce.

Thus, if in FIG. 4, the reticle spacing A is such to reinforce the spacefrequency f, (corresponding to the null n.;) in FIG. 3, and the powertransmitted through the reticle analyzed, it will be found that asubstantially greater amplitude of power modulation will be effected bythe chopping action of the reticle in the presence of a target 30 thanin its absence. This applies, of course, when the target image issubstantially of the same dimension as the reticle aperture spacing,namely A/2.

In order to better understand the disadvantages of the prior art and themanner in which the present invention overcomes them, the generalarrangement of the component parts of a prior art radiant energy systemmust be considered. Such a system is shown in FIG. 5 including adetection assembly 41, a mechanical drive 42 including horizontal andvertical gimbal supports for the detection assembly 41, a search program43 to move detection assembly 41 in a predetermined manner for searchinga selected surveillance volume, a filter 44 connected from the output ofdetection assembly 41, a detector 45 connected to the output of filter44, and a recorder 46 connected from the output of detector 45 forrecording signals received during search.

Detection assembly 41 includes radiant energy collection apparatus 47 toproject a circular imaged field of view 48 on the symmetrical center ofa reticle 49 having a gear 50 mounted thereon. The reticle 49, as shownin more detail in FIG. 6, comprises a substantially planar disc defininga plurality of alternate angularly disposed areas of relative opacityand transparency to the intensity variations throughout an imagerepresenting the field of view of an apparatus such as 47. By way ofexample, in FIG. 6, the relatively transparent areas are indicated at57, while the relatively opaque areas are indicated at 58. Thesector-like areas may be thought of as extending from a smaller radiusto a greater radius from the symmetrical axis or center 67 of thereticle. The transparent areas 57 of the reticle can then be seen toform apertures through which radiant energy may pass, and, hence, thetransparent areas 57 may be considered as the reticle aperturescomparable in function to the apertures 38 of reticle 37 in FIG. 4. Inthe prior art arrangement of FIG. 5, it can be seen that the opticalaxis 47a, of the optical collection apparatus 47, is coincident with theaxis 67 of reticle 49. Reticle 49 is rotated about its symmetricalcenter by a spin drive motor 51 which rotates a gear 52 on a shaft 53,gear 52 meshing with gear to rotate reticle 49 to which gear 50 isfixed. Radiant energy passed through reticle 49 is focused on a radiantenergy sensitive cell 54 by means of a lens 55. The output of cell 54 isthen connected to filter 44.

It is to be noted that in connection with the reticle 37 of theinvention shown in FIG. 4, the transparent areas 38 are equal in widthto opaque areas 56 and are at all points on the reticle spaced the samedistance apart. Thus the alternate opaque and transparent areas aresubstantially equal in size. This means that the fundamental of thealternating current signal or carrier produced by reticle choppingaction will be as large as it is possible to produce in relation to theamplitude of its harmonic components. However, on a circular disc typereticle, the transparent areas cannot be spaced an equal distance apartat all points on the reticle because the circumferential separation ofthe transparent areas and their dimension are variables depending uponthe radius r, at which they are examined. For this reason, therelatively transparent and opaque areas respectively indicated by theindices 57 and 58 associated with reticle 49, in FIG. 6, are sector-likein shape, and are substantially equal in size. It is to be noted thatthe arcuate distances indicated at s and s on reticle 49, in FIG, 6,are, therefore, different. This means that the size discriminationcharacteristic of reticle 49 shown in FIG. 6 is not nearly as good asthat of the reticle 37 shown in FIG. 4. That is, background objects of asize s /2 would be chopped with the same efficiency at the perimeter ofreticle 49 as a target of a size s /2 would be at the radius of s,indicated in FIG. 6. Thus, the reticle 37 shown in FIG. 4 may beconsidered as a space filter substantially more selective than the spacefilter comprising the conventional spoke-type reticle 49 shown in FIG. 6for the reason that the reticle chopping action of reticle 49 shown inFIG. 6 reinforces a plurality of fundamental space frequencies whereasthe chopping action caused by movement of reticle 37 in a direction 40,indicated in FIG. 4, reinforces only a single fundamental spacefrequency.

A search procedure which may be employed in the control of detectionassembly 41, by search program 43, through mechanical drive 42, shown inFIG, 5, is illustrated in FIG. 7 where the field of view 59 is shown tobe initially positioned at a time t, in the upper left-hand corner of apredetermined optical frame. For convenience in description, the leadingedge of the field of view is, at this position, designated by index 1This frame is indicated by the dotted line rectangle 60. By propercontrol of the mechanical drive 42 by search program 43, shown in FIG.5, the field of view 59 may be made to systematically scan the frame 60.The manner in which this systematic scanning of the frame 60 isundertaken may follow various patterns. By way of example, in FIG. 7,the leading edge of the field of view 59 is, at time t,, positioned asindicated and moved from left to right so that at time t the field ofview 59 is at the right-hand extremity of frame 60. During the intervalfrom time 1 to time the field of view is moved downwardly along a curvedpath P so that its leading edge is at the position shown at time 1Thereafter, the field of view moves from right to left to the positionindicated at time 1 This systematic pat- .tern of scan, generallyindicated by the dotted line 60' (with arrows V thereon indicating thevectorial direction of scan velocity) is continued until the entireframe 60 has been examined. Purely by way of example, in theillustration of FIG. 7, the subject matter embraced by the field of viewtime 1 is shown to correspond to that indicated in FIG. 1(b). Thehorizon line 28 of FIG. 1(b) is, in FIG. 7, shown to a fuller extent,however, and can be seen to be of a length many times greater than thatportion of it embraced by the field of view.

Signals which may be produced at the output of filter 44 shown in FIG. 5during the search operation indicated in FIG. 7 is shown in FIG. 8. Atsome time such as the position of the leading edge of the field of viewwill be coincident with the position indicated at I in FIG. 7, and thefield of view will be moving from right to left as indicated by thearrow on dotted line 60' adjacent this position. At this instant, itwill be assumed that the aircraft will not have as yet been encounteredby the moving field of view. The content of the field of view at thisinstant will, of course, be in the process of analysis by rotatingreticle 49, and there will be some background content within the imagedfield of view having a space frequency description causing a relativelylow amplitude alternating current carrier signal (the fundamental ofwhich corresponds to the chopping frequency of the chopping reticle) toappear at the output of filter 44 shown in FIG. 5. This is generallyindicated by the low amplitude portion 85;, of the alternating currentcarrier signal depicted at 85 in FIG. 8(a). Still at a later time, 1 thefield of view will have been lowered somewhat and now moving from leftto right although not as yet having encountered the aircraft. The outputof the cell will then be relatively low such as the previous level 85However, as soon as the leading edge of the field of view encounters theaircraft 30 (such as at time 1 the amplitude of the carrier signalappearing at the output of filter 44, will increase, by a substantialamount, to an amplitude illustratively indicated in FIG. 8(a) at 85 Theamplitude of the carrier 85 rises to the value 85;; for reasonshereinabove set forth, namely, the reticle apertures have been sodimensioned so as to reinforce a plurality of space frequenciesincluding a space frequency at which substantial power is contributed byobjects whose images have a size substantially corresponding to theimage size of the aircraft being sought. It is under these conditionsthat the percentage modulation of the total energy passing through thereticle, by virtue of the chopping action of the moving reticle spokeson the targe image, will be maximized. This increase in the ampltiude ofthe carrier produced at the output of filter 44 will continue for aduration of time corresponding to the length of the time that the target30 remains within the moving field of view. This has been illustrativelyshown in FIG. 8 to be for a period of time r to 1 which period is termedthe dwell" period of the object or target within the moving field ofview.

The envelope of the carrier modulation indicated in FIG. 8(a) isderived, as shown in FIG. 5, by means of the combined action of filter44 and envelope detector 45. At the output of detector 45 shown in FIG.5, there will appear an alternating current signal of the charactershown at 86 in FIG. 8(b). Here a portion 86 corresponds to the ampltiudeof the carrier in FIG. 8(a), likewise portion 86;; correspondsrespectively to the carrier at amplitude 85 in FIG. 8(a).

The change in amplitude of the envelope 86 to the value 86 upon thefield of view occasioning a target, therefore represents a relativelylarge percentage increase in the value of the detector output signal 86.

With the prior art, centered reticle arrangement depicted in FIG. 5, andduring the searching operation shown in FIG. 7, if a target appearsabove the center of the field of view, and, therefore, above the centerof the reticle 49 in traversing the path, for example, as indicated at60' in FIG. 7, and the reticle is spinning clockwise as taken lookingout through the reticle toward the field of view, it will be seen thatthe direction of reticle movement above the center will be in the samedirection as the scan vector depicted by arrow V between times 1 and rThe arrow V indicates the vector direction in which the field of viewmoves in foilowing out the search program. Conversely, if a targetappears below the center of the field of view 59 during its excursionalong the line 60, the direction of reticle movement at the particluartarget position considered above will be opposite the direction of scanvelocity V Thus, a Doppler frequency shift in the value of the carrieris produced of a magnitude depending upon the position in which a targetexists in the instantaneous field of view 59. For this reason, thebandwidth of filter 44, in FIG. 5, must be rather large to pass outputsignals due to targets existing both above and below the center ofimaged field of view or on opposite sides of a line extending throughthe center of the instantaneous field of view 59 in the direction ofscan velocity. This is one of the serious disadvantages of the prior artconventional spoke-type reticle arrangement shown in FIG. 6 wherein thecentered imaged field of view 48 is projected thereon by radiant energycollection apparatus 47 shown in FIG. 5. Thus, if the ideal reticlearrangement of the invention is employed, the direction of movement ofall reticle apertures over the imaged field of view will be the samethroughout the imaged field of view. For this reason, regardless of theposition of a target in the imaged field of view, the Doppler frequencycreated by given vectorial values of scanning and reticle choppingmotions will be the same. For this reason, a band pass filter employedin place of filter 44, in accordance with the invention, may be madesubstantially more selective.

Thus far, it has been explained how the reticle arrangement of theinvention provides improved space filtering and provides means by whicha more selective electrical filter may be employed to improve theoperation of the radiant energy collection apparatus. As will be broughtout more fully hereinafter, the present invention in one of its formsmay be used in either searching or tracking. Still further, the Dopplerfrequency shift problem of conventional spoke reticles such as reticle49 having the centered imaged field of view 48 thereon is a disadvantageonly in search in the prior art. Hence, the advantages of the inventionin overcoming problems of frequency shift relate only to the searchoperation. The remaining advantages of the invention describedspecifically hereinafter over prior art apparatuses relate specificallyto problems which arise with prior art radiant energy detection systemsduring tracking operations.

The mechanical structure of conventional prior art detection assembly 41shown in FIG. 5 may be as indicated in FIG. 9 having a mechanical driveincluding an elevation drive 112 for elevation gimbal supports 113, anelevation data pick-off 114, an azimuth drive 115 for azimuth gimbalsupports 116, and an azimuth data pickofi 117. The outputs of the datapick-offs 114 and 117 are converted in computer 118 to a referencesignal which is impressed upon a phase comparator 119 to produce anoutput signal 6 proportional to the angular displacement of a target orairplane 120 shown in FIG. 9. The angle 0 is shown as measured in aplane perpendicular to the axis of nutation 121, and is the angle, inthis plane, between a reference line 121' and a straight line extendingfrom the axis 121 and the target itself. The reference line 121' isgenerally established as horizontal. Phase comparator compares thereference signal output of computer 118 to the output 01' a radiantenergy sensitive cell,

not shown, in detection assembly 110 which passes through a filter 122to both phase comparator 119 and a frequency discriminator 123.Frequency discriminator 123 produces an output signal proportional tothe magnitude of the polar coordinate r of the position of airplane 120from a point of origin indicated at zero in FIG. 9. The coordinates rand are impressed upon a computer 124 having a clock motor input 125 toimpress output signals on a servo mechanism 126 to control elevation andazimuth drives 112 and 115 to cause nutation of the detection assembly110 about airplane 120. Nutation of detection assembly 110 is indicatedby four dotted circles 127, 128, 129 and 130 which represent the fieldof view of detection assembly 110 at successive quarter cycles ofnutation. Due to the nutation of the detection assembly 110, only imagesof objects in area 131 will exist in their entirety throughout anutation cycle. The center of the field of view will move on an ellipseindicated at 132. Obviously, all of the dotted lines indicated in FIG. 9are actually circles when viewed along the reference axis 121, but theyare drawn as ellipses due to the perspective view taken.

From the foregoing explanation, it will be understood that all radiantenergy transmitted to detection assembly 110 will be chopped. Asexplained previously, only the energy received in the area of circle 131will be received all the time during the one complete cycle of nutation.This means that all radiant energy received through the areas of theinstantaneous imaged fields of view 127, 128, 129 and 130 not commonwith the circle 131 will vary with time. Thus, if the background is notuniform, scanning noise of the same frequency as the cyclic nutationfrequency will be created which will make it more difiicut todistinguish a target signal at maximum range. The prior art system shownin FIG. 9 also suffers from several other disadvantages. In the firstplace, due to the fact that the instantaneous field of view is movedduring tracking, the minimum resolution of the radiant energy collectionapparatus employed therewith will vary with nutational position. Thatis, aberration caused by the use of mirrors or lenses necessarily causesthe size to which an image of the target may be resolved to change withthe nutational position of detection assembly 110. This is also anothersource of undesirable scanning noise during tracking. The maximumresolution of any radiant energy detection system employing mirrors orlenses at a maximum range is called the blur circle size. Thus, to statethis same disadvantage another way, the blur circle size varies with thedistance r of airplane 120 from axis of nutation 121. Still further,this change in size means that a single reticle of a given aperturewidth cannot optimize space frequency reinforcement since effectivetarget size or blur circle size will change during the nutation oftracking with prior art apparatus.

The large size of the detection assembly 110 shown in FIG. 9 requiresrelatively large component parts for structural rigidity. Still further,a relatively large amount of electrical power must be employed to nutatethe detection assembly 110.

The output of frequency discriminator 123 will not be a linearrepresentation of the angular position of airplane 120 represented bythe angle between a line through airplane 120 and the gimbal center ofdetection assembly 110 and the axis of nutation 121. This is truebecause chopping action obviously does not take place during a portionof the nutation cycle when airplane 120 falls outside of circle 131.

All the above-described disadvantages of the prior art are overcome bythe present invention by employing a detection assembly of the generalform indicated in FIG. 10. This embodiment of the invention comprisesoptical collection apparatus 73 having an optical axis 72. Opticalcollection apparatus 73 projects an imaged field of view upon a reticle69, the imaged field of view being indicated at 52. An integrating lenselement 77 collects energy passed by the reticle 69 and directs thisenergy to a sensitive cell 78. The output terminal of the cell isdesignated 79. The reticle 69 is adapted to be spun about its axis 67 bymeans of a spin drive motor 70.

In accordance with the present invention, the spin drive motor 70, whoseshaft is connected to the axis of reticle 69, is adapted to be drivinglypositioned around a closed path surrounding the optical axis 72. To thisend, the spin drive motor 70 is mounted on a tubular member 95 to whichis attached a drive gear 96. An actuating gear 97, attached to the shaftof a planetary drive motor 98, engages the drive gear 96 so as to permitthe spin drive motor 70, and hence, the axis 67 of the reticle, to bepositioned at any point along the dotted line path X. A drive powersupply 102 impresses a control voltage on planetary drive motor 98.Similarly, a spin drive power supply 102 is connected to spin drivemotor 70. The output of spin drive power supply 102, is coupled to spindrive motor 70 through a slip ring and brush assembly, whose componentsare generally indicated by reference numeral 108. In order to obtain areference signal input for phase comparator 119 and frequencydiscriminator 123 of FIG. 9, a reference signal generator 103 shown inFIG. 10 may be provided. Reference signal generator 103 is mechanicallycoupled by means of a gear 104 to the actuating gear 97. The outputsignal of reference signal generator 103 may be alternating signal of afrequency equal to the cyclic frequency of movement of tubular member 95through 360 mechanical degrees.

Optical collection apparatus 73 may be conventional and is indicated inFIG. 11. The general form of optical system shown in FIG. 11 is commonlycalled a Cassegrain optical mirror system. It incorporates afrustoconical frame structure 21C in which primary and secondary mirrors22C and 23C are mounted. Infrared radiation is reflected from mirror 22Cto 23C and then into an aperture in a reticle assembly 160. Collectionapparatus 73 is hinged at 24C to gimbals 25C, which have illustrativelybeen broken away for clarity.

Reticle assembly 16C, shown in more detail in FIG. 12, may incorporate areticle 17C which may be driven by a motor M that is maintained in afixed position relative to framework 21C. Motor M is provided withelectrical leads 27C. A light baffle 28C (FIG. 11) is lo cated in thecenter of secondary mirror 23C to prevent infrared radiation fromentering an aperture in the forward portion of reticle assembly 16C withthe exception of infrared radiation reflected from secondary mirror 23C.An infrared cell, not shown, is located in a Dewar flask 29C in which itis cooled. The output of the cell is converted into an amplifiedelectrical signal which is impressed upon output leads 30C.

A mechanical drive including motor M which may be maintained in a fixedposition relative to a reticle housing 31C and framework 21C is shown inFIG. 12. Similarly, an internal gear wheel 320 may also be maintained ina fixed position relative to housing 31C, gear wheel 32C being annularin shape and having radial teeth 33C extending inwardly.

Reticle 17C is provided with a shaft 34C about which it is rotated. Thisshaft is rotatably mounted in a reticle supporting body 350 to which agear 36C is fixed by means of screws 37C. Gear 36C is driven by a pinion38C fixed to the drive shaft of motor M. An idler gear 40C is mounted ona shaft 41C which is rotatably mounted in reticle supporting body C.Shaft 41C is rotatably mounted in reticle support member 35C in bearings42C. Idler gear C has teeth to mesh with both the teeth of gear wheel32C and a gear 43C which is rotatably mounted on shaft 34C that isjournaled in bearings 44C and 45C. Reticle supporting body 35C is thenfixed to a field stop 460 by means of screws 47C. Shaft 34C is, ofcourse, fixed to reticle 17C, as stated previously, and movement of gear43C therefore rotates reticle 17C about the axis of shaft 34C. However,movement of 15 gear 36C also produces movement of the axis of shaft 34Cabout the optical axis of the system. By use of the idler gear 40C, theaxis of shaft 340 is rotated about the center of the imaged field ofview in the same direction that reticle 17C itself rotates about theaxis of shaft 340.

Reticle support member 35C is rotatably mounted in an annular ring 49Con ball bearings 50C, rings 49C being maintained in a fixed positionrelative to housing 316 by means of screws 51C. Similarly, field stop46C is rotatably mounted in housing 310 on ball bearings 52C.

In FIG. 13 the manner in which the reticle assembly is incorporated in aradiant energy search system is illustrated including radiant energycollection apparatus 200, a radiant energy sensitive cell 201 to receiveenergy collected by the apparatus 200, a filter 202 to receive theoutput signal of cell 201, and a discriminator 203 to receive the outputsignal of filter 202. Discriminator 203 impresses its output signal bothon a phase detector 204 and an amplitude detector 205. Phase detector204 also receives an input signal from a reference signal generator 206.Phase detector then impresses an output signal upon servo means 207which also receives an output signal from detector 205. Servo means 207controls the position of energy collection apparatus 200.

As explained previously in connection with FIG. 9, frequencydiscriminator 122 produces an output signal r proportional to thedistance indicated at r from the origin to the airplane 20 in an imagedfield of view on the reticle employed in energy collection apparatus200. Phase detector 204 produces an output signal 0 proportional to theangular position of the image of the aircraft 120 on the reticleemployed in energy collection apparatus 200. The outputs of both phasedetector 204 and amplitude detector 205 are therefore direct-currentvoltages which may be amplified, if desired, and impressed on a resolverto convert the polar coordinates r and 6 into Cartesian coordinates.Azimuth and elevation drive motors may then be operated to correct therespective azimuth and elevation orientation of energy collectionapparatus 200 so that the image of airplane 120 on the reticle thereinis at the center of the circular field of view. From the foregoing, itwill be appreciated that all that is necessary to be included in servomeans 207 is a resolver and azimuth and elevation drive motorsillustrated diagrammatically in the prior art FIG. 9 as azimuth drive115 and elevation drive 112 respectively. Servo means 207 may be of thesimplest servo type and need not even require any azimuth data orelevation data pick-off such as the data pick-offs 117 and 114,respectively, shown in the prior art FIG. 9, because when an image ofthe aircraft 120 on the reticle and energy collection apparatus 200 isat the center of the circular field of view thereon, the outputs ofphase detector 204 and amplitude detector 205 will become zero and theazimuth and elevation drive motors no longer will be operated to changethe position of energy collection apparatus 200.

Energy collection apparatus 200 may be identical to the collectionapparatus 73 shown in FIG. 11. Radiant energy sensitive cell 201 andreference signal generator 206 may be identical to cell 78 and generator103 shown in FIG. 10. It is to be noted that cell 201 and referencesignal generator 206 will be mechanically mounted on the structure ofenergy collection apparatus 200, the same being shown separately in FIG.13 where input signals to filter 202 and phase detector 204 initiate.Filter 202 is a carrier frequency filter which may be identical to thefilters illustrated at 44 in FIG. and at 122 in FIG. 9. Discriminator203 may be identical to frequency discriminator illustrated at 123 inFIG. 9. Phase detector 204 may be identical to phase comparator 119illustrated in FIG. 9. Amplitude detector 205 may be simply a diodedetector. In regard to FIG. 13 in general, it is to be noted that eachand every one of the component parts or circuits illustrated therein maybe en tirely conventional with the exception of energy collectionapparatus 200 which incorporates the specific reticle position and drivearrangement in accordance with the invention. In this same regard,either the arrangement of the invention shown in FIG. 10 or thearrangement of the invention shown in FIG. 12 may be employed, thedifference between the same being that spin drive motor 102 is providedin addition to planetary drive motor 98 shown in FIG. 10 whereas asingle drive motor M is provided in FIG. 12 incorporating a planetarylike gear system not only to rotate reticle 17C about its axis ofsymmetry, but also to cause its axis of symmetry to describe a cylinder.

From the foregoing, it will be appreciated that the output of phasedetector 204 will be a direct-current voltage proportional to theangular position 0 indicated in FIG. 9 in connection with the positionof airplane 120 on the reticle of the invention and the output ofamplitude detector 205 will be a direct-current voltage proportional tothe distance from the origin 0 to the image of the airplane 20 on thereticle of the invention. The azimuth and elevation drive motors arethen operated from a resolver to change polar coordinates r and 6 toCartesian coordinates to reposition energy collection apparatus 200until the image of the airplane is on the origin, that is, at the centerof the circular field of view on the reticle of the invention.

As probably already is apparent, the use of the terminology to describemovement of the reticle of the invention in the image field as scanningthe image of the field of view in substantially the same directionrefers generically not only to the movement of reticle of the type 37illustrated in FIG. 4, but also of the type illustrated at 69 in FIG. 10and refers to the fact that the transparent and opaque areas of thereticle, at least in part, move completely out of and back into the areaof the field of view for example illustrated at 52' in FIG. 10. Thus,the terminology describing a reticle movement with the areas scanningthe image of the field of view in substantially the same directiondistinguish over the prior art embodiment illustrated in FIG. 5 whereabove a horizontal line through the origin 67 of the field of view 48moves to the right and the reticle below a horizontal line through theorigin 67 moves to the left when the reticle 49 is rotated to the rightas viewed in FIG. 5.

The reticle employed with the invention may be the conventionalspoke-type reticle as described previously. However, due to the factthat imaged field of view may be projected on a conventional spoke-typereticle in accordance with the invention near the periphery thereof, ifthe diameter of the imaged field of view projected thereon is rathersmall in comparison to the diameter of the reticle, it will beappreciated that the variation in width of transparent reticle aperturespassing through the imaged field of view may be very small.

From the foregoing, it will be appreciated that the method and apparatusof the invention provide means for improved space filtering. Due to thefact that, in accordance with the invention, the imaged field of viewmay be projected onto a reticle at a position near the periphery thereofspaced a substantial distance from the center, the aperture widthvariation with radius on the reticle within the imaged field of viewwill be substantially smaller than in apparatus of the prior art whereinthe imaged field of view is located at the symmetrical center of thereticle.

Thus, space filtering is improved both in searching and in trackingoperations. In search only, the Doppler frequency shift effect of priorart apparatus is substantially reduced by the present invention due tothe fact that all the reticle apertures pass through the imaged field ofview, in accordance with the invention, in substantially the samedirection. In search, it is contemplated that the periodic planetarytranslation of the reticle axis be suspended as by conditionallywithholding the application of power to the planetary drive motor 98 inFIG. 10. The arrangement of FIG. 12 is, of course, primarily useful intrack.

It is also an outstanding feature of the invention that tracking may beperformed with the optical axis maintained substantially stationary.That is, nutation is not required. Due to the fact that the optical axisis substantially stationary, blur circle size will not change rapidly orsubstantially during tracking because when nutation is avoided inaccordance with the invention, a target is not viewed at differentangles through the optical collection apparatus. Scanning noise is notencountered since nutation or scanning is not performed in the trackingoperation of the present invention. Still further, the linearity of theoutput signal is improved since a target cannot fall outside the fieldof view of the apparatus as with periodic nutation.

Since nutation is avoided, it is also unnecessary to mechanically movethe detection assembly equipment rapidly and hence the advantages oflighter structure having less rigid load requirements may be employedand this structure may be moved with a substantially less electricalpower. It is to be noted that in manual or automatic tracking, thedetection assembly of the invention will, of course, be moved to track atarget. However, this movement will be substantially slower than thetracking mutation movement of the apparatus of the prior art.

What is claimed is:

In an electro-optical system for determining the position of a source ofradiant energy, said system having means for collecting a portion ofenergy radiated from said source and a cell positioned on the opticalaxis of the collection means for producing an electrical signalproportional to the intensity of radiant energy projected thereon bysaid collection means, the combination comprising: a disc-shaped reticledisposed between said collection means and said cell, said reticleincluding spaced bodies of relative opacity to radiant energy, saidreticle being positioned to interrupt radiant energy projected onto thecell by rotation thereof such that all of said bodies pass substantiallyin the same direction through the path of radiant energy, said bodieshaving the shape of radial spokes extending approximately from thecenter of said reticle to the border thereof; first .power means forrotating said reticle about its symmetrical axis at a substantiallyconstant speed; second power means for translating the position of theaxis of said reticle in a circle around the optical axis of saidcollection means at a substantially constant radius from the opticalaxis of said collection means; said reticle being both rotated about itsaxis and translated about the optical axis of said collection meanssimultaneously; filter means connected to said cell for deriving analternating signal of a frequency dependent upon the relative speeds ofrotation and translation of said reticle; a reference signal generatorfor producing an alternating signal having a period equal to the timeduring which said reticle is translated once around the optical axis ofsaid collection means by said second power means; a discriminator fordetecting the frequency modulation of the output of said filter means; acomparator producing an output signal proportional to the differencebetween the phases of the output signals of said reference signalgenerator and said discriminator; a detector for producing an outputsignal proportional to the amplitude of said discriminator outputsignal; and servo means responsive to the output of said comparator andsaid detector for positioning said collection means at an angle suchthat the optical axis thereof is directed toward a target received bysaid system.

References Cited by the Examiner UNITED STATES PATENTS 1,122,924 12/1914Henderson 74-785 1,626,719 5/1927 Callison 74-785 2,421,012 5/1947 Chew244-143 2,431,510 11/1947 Salinger 244-143 2,878,396 3/1959 Behm et al88-1 2,892,124 6/1959 Rabinow 250-233 X 2,905,828 9/1959 OMaley 88-12,906,883 9/1959 Hansen 244-143 2,942,118 6/1960 Gedance 244-1432,949,672 8/1960 Ostergren 250-233 2,981,842 4/1961 Kaufold 250-8332,981,843 4/1961 Hansen 250-203 3,134,022 5/1964 Jones ct al 250-203 X3,143,654 8/1964 Aroyan et al 250-203 X FOREIGN PATENTS 68,298 11/ 1957France.

RALPH G. NILSON, Primary Examiner. CHESTER L. JUSTUS, M. R. WILBUR,Examiners.

D. G. BREKKE, R. A. FARLEY, W. STOLWEIN,

Assistant Examiners.

